Biodiesel is produced commercially from vegetable oils and/or animal fat through transesterification with alcohol to convert triglycerides into alkyl esters of the fatty acids (biodiesel) and glycerol using a basic homogeneous catalyst, such as sodium hydroxide, potassium hydroxide, and sodium acetate. Because base-catalyzed reactions are very sensitive to the presence of free fatty acids (FFAs), they are unsuitable for unrefined oils with FFA contents higher than about 3%. Moreover, to prevent saponification during the transesterification reaction, FFA and water contents in the oil feed should be less than 0.5 and 0.05 wt %, respectively, which means that essentially only pure vegetable oils can be used. High FFA levels in the oil feed also deplete the base catalyst through acid-base neutralization reactions. Base-catalyzed reactions also require that the NaOH catalyst be neutralized with acid and removed from the reactor effluent with a water wash. The resulting salt byproduct from the acid/base neutralization must then be separated from the biodiesel product. Finally, water produced in the reaction promotes saponification of the fatty acids with the NaOH catalyst which renders phase separations among the products (biodiesel and glycerol) and the salts even more difficult.
Homogeneous acid catalysts have also been employed in transesterification reactions for biodiesel production but acid-catalyzed reactions are slow and generally less suitable for large scale operations. Although the performance of the acid catalyst is not affected by the presence of FFA in the oil or fat feedstock, nevertheless, the process requires a high alcohol-to-oil mole ratio and long reaction times due to the low activity of the acid catalyst. When using a base or an acid catalyst, the transesterification process, which occurs in a corrosive environment, requires costly neutralization, water wash, filtration, and solid waste disposal steps to remove the spent catalyst from the biodiesel and glycerol product streams.